1. Field of the Invention
The present invention relates to object sorters and counters such as tablet or pill sorting or counting devices. More particularly, the invention relates to a method and apparatus for controlling the amplitude of oscillation and the feed rate of an oscillating object sorter or counter.
2. State of the Art
Object sorters and counters including those using oscillation or vibratory motion are well known in the art. These types of devices all share the common goal of reducing a collection of discrete objects to an orderly line of flow so that they may be sorted and/or counted as they move past one or more optical sensors. Such devices take various forms including rotational and linear vibrators, rotating discs, air jets, gravity feeds, moving belts, etc. The vibrating devices generally include an input hopper or bowl and various funnels, chutes, or channels, one or more of which are vibrated by vibrator coils so as to direct the objects into one or more single-file lines of flow. It is recognized that the amplitude of vibration is important in controlling feed rate and that a controllable feed rate is desirable.
Several methods and devices are now used to control the feed rate of vibrating object sorters and counters. A typical manner of controlling the feed rate in the prior art is to manually adjust and set a controller that will send pulses of constant duration to a vibrator. Such a manual adjustment will result in a constant rate of vibration and a constant feed rate so long as other conditions affecting feed rate remain constant.
A typical controller is shown in prior art FIG. 1. When power line voltage is applied across nodes L1 and L2, and switch S1 is closed, current tries to run through the load (vibrator coils) which is connected to nodes H1 and H2, but is blocked by a silicon controlled rectifier (SCR) which has not been triggered. When the voltage at node L1 is positive, a trickle current flows through resistor R1 and user adjustable potentiometer R4, tending to charge capacitor C1. Diode D1 is chosen not to conduct below a nominal 8 volts and diode D2 is reverse biased. Therefore, the charging rate of capacitor C1 is determined by R1 and the manual setting of R4. D4 is a four layer diode which not only blocks reverse current flow, but also blocks forward current until a critical voltage is reached. At the critical voltage, D4 acts a short circuit and it will continue to conduct as long as current is supplied. Therefore, capacitor C1 continues to charge until the voltage across it (and across D1) reaches approximately 8 volts. Upon reaching 8 volts, D1 becomes a closed switch and C1 discharges (through D1, R2, R3, and the SCR gate circuit) to the "break-back" voltage of D1. Since not enough current can flow through R1 and R4 to keep D1 conducting, it recovers when C1 is discharged. The discharge current from C1 flows through R2 (which limits it to about 80 milliamperes) and splits between R3 and the gate (triggering) circuit of the SCR. This fires the SCR which then "holds" its conductivity after the trigger is gone. The SCR continues to conduct until its forward current drops below its minimum holding value. Since the supply for the triggering circuit is the voltage developed across the SCR, this supply disappears when the SCR turns on, preventing C1 from recharging during the rest of the positive half cycle. When L1 is negative, D2 becomes forward biased shunting C1 and preventing its charging to a negative voltage. Thus at the beginning of each positive half cycle, the voltage across C1 starts near zero and rises toward +8 volts. The sooner it reaches +8 volts, the longer the SCR conducts and consequently, the higher the vibration rate (and hence feed rate). The values of C1, R1, and R4 are chosen so that the SCR cannot fire much before the 90 degree phase angle of the power line. Varistor R5 absorbs voltage spikes to protect the SCR. A minimum holding current resistor R6 shunts the vibrator coils. When the SCR is triggered, its current must rise above the holding current value before the trigger pulse dies out. If the inductance of the vibrator coils does not let the current rise fast enough, R6 provides the needed extra holding current.
As mentioned above, at a given setting, the known controllers such as shown in FIG. 1 provide for a constant rate of pulses to the vibration coils and therefore the system provides a constant feed rate so long as other conditions affecting the feed rate remain constant. Unfortunately, other conditions affecting feed rate do not remain constant and the user must frequently re-adjust the controller if optimal feed rate is desired. Conditions which affect the feed rate include the feeder load, the type of objects being handled, and the condition of the feeder itself. Mechanical variations in the feeder result in varying responses to an otherwise constant power input. For example, as the equipment ages, parts of the feeder wear. Fastener clamp forces relax, springs fatigue, etc. These mechanical changes in the feeder cause it to vibrate and thus feed at a different rate even though the amount of power applied to the vibrator coils remains constant. Another important factor which affects feed rate is temperature. As the feeder operates, the vibrator coils dissipate heat, heating the driver and changing the spring rates of the springs. This changes the amplitude of vibration for a given power input.
Servo controllers such as the one shown in prior art FIG. 2 have been applied to vibrators to correct for the conditions which tend to vary the vibration amplitude. The basic functioning of the servo controllers are the same as the aforementioned controllers except for the elimination of the adjustment potentiometer and the substitution of a lower valued resistor in the C1 charging circuit. This results in a tendency for the SCR to run wide open. The output of the controller is reduced to operational levels by adding a conductor across nodes B1 and B2. This diverts some (or all) of the charging current to capacitor C1 and thus delays the firing time of the SCR. If the conductor across B1 and B2 is a closed switch, the SCR will not fire at all. For normal running, a Photoelectric Transducer (PET) is connected across nodes B1 and B2. The conducting element in the PET is a photo-transistor D whose conductivity is varied by the amount of light conveyed from a juxtaposed light-emitting-diode (LED), powered from terminal E. When the full illumination of the LED falls on the photo-transistor, it conducts enough to turn off the controller completely. When the photo-transistor is darkened, the controller runs full on, thus providing control over the entire output range of the controller.
As shown in prior art FIGS. 3 and 4, the PET is attached to the non-moving end-plate of a vibrator V, and a shutter S is attached to the moving end-plate and located so that it can block the light from the LED of the PET. The shutter is set so that it blocks the light when the vibrator is standing still. Thus, when the controller is turned on, it tends to run wide open. This in turn causes the moveable end-plate of the vibrator to move the shutter and allow light to strike the phototransistor, which cuts down the drive. Equilibrium is established in one or two cycles of the power line frequency. At equilibrium, the vibrator vibrates at the power line frequency and the shutter covers and uncovers the photo-transistor once each cycle. When the power line voltage crosses the zero-axis (going positive), the photo-transistor is dark and C1 is charging rapidly. Later in the cycle, the shutter starts to uncover the photo-transistor and the charging of C1 is slowed. The resultant late firing of the SCR cuts down on the vibratory amplitude. For low amplitude operation, the shutter is mechanically set so that it barely covers the photo-transistor and a small excursion of the vibrator is sufficient to cut back the drive. For high amplitude operation, the shutter is mechanically set so that it overlaps the photo-transistor by a wider margin, thus forcing a larger excursion before the regulating action of the photo-transistor becomes effective. Transistor, Q1 connected across nodes B1 and B2, in normal operation, is biased to a non-conducting state and has no effect. Turn-on current is provided through Zener Diode Z1, which does not conduct below a nominal 5.1 volts. Since it derives its voltage source across the LED in the PET, and since normal voltage across the LED cannot exceed 1.7 volts, it is never high enough to pass current through Z1. If the LED is unplugged, disconnected or burned out however, the voltage input to Z1 exceeds 5.1 volts, Z1 conducts, and Q1 turns on. This robs C1 of all of its charging current and shuts off the controller.
The known servo controllers such as the one described above with reference to FIG. 2 have an important disadvantage. It is recognized that the position (exact edge location) of the shutter is critical for correct control of the vibrator. The position of the shutter edge, however, is subject to misalignment by shocks that might occur for instance during shipment of the equipment and even by the operation of the equipment. As a result, the vibration and feed rates are undesirably altered from initial settings. This problem is partially overcome in the art by incorporating a feedback system which adjusts the vibration rate as a function of the product counting rate. However, it will be appreciated that prior to the product being sensed and counted, vibration amplitude is uncertain, resulting in unoptimized feed for the initial portion of the product flow stream and hence potential counting inaccurracies. This problem is especially acute where critical tablet image data is collected by a sensor array at the start of a feed run. As described in the parent application, such critical data is used to arrive at an accurate count during the rest of the feed run. Failure to start the feed run at the desired rate thus jeopardizes the accuracy of the entire run.